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I've talked HERVs on a few occasions on this blog, in relation both to the autism spectrum conditions (see here) and myalgic encephalomyelitis (ME) (see here).
Without getting too technical, the crux of those posts was to suggest that although those bits of virus in our genomes are not necessarily 'active viruses' (i.e. not able to produce infectious virus or replicate) they may have implications for things like autoimmunity given that HERV proteins are supposedly recognised as 'self' by the almighty MHC  and seem to be able to provoke autoimmunity (in mice) . That and the fact that the expression of HERVs may be kept in check by epigenetic means (methylation)  and 'hypomethylation [of DNA] = more genomic instability'  so, theoretically under certain circumstances could mean HERVs get a chance to start expressing (something). Or that's the theory (I think)...
With all that in mind, I move to the paper by Balestrieri and colleagues  (yes, the same group who completed the HERV paper with autism in mind) who discussed some very preliminary data on the expression of certain families of HERVs. Indeed, they reported: "The expression levels of HERV-H are significantly higher in patients with ADHD [attention-deficit hyperactivity disorder] compared to healthy controls".
I can't pretend to know all the ins-and-outs of how one goes about assaying for the expression of HERVs - "expression of retroviral mRNAs from the three HERV families was evaluated in peripheral blood mononuclear cells (PBMCs)" apparently. It was however interesting to see that HERV-H 'over-expression' was "significantly higher in patients with ADHD compared to healthy controls". HERV-H was also the same family reported to be 'more abundantly expressed' in cases of autism.
A search of some of the literature covering HERV-H reveals that it is a gamma-retrovirus (yes, similar to those letters X-M-R-V and that de-discovery issue). In terms of associations and roles, it "contributes to pluripotency in human cells" according to the paper by Santoni and colleagues  and their finding of high levels of HERV-H RNA in human embryonic stem cells. The HERV-H family have also been suggested to have immunosuppressive properties .
HERV-H has been linked to conditions such as multiple sclerosis as per papers like this one by Christensen  which also hinted at how other viruses may 'interact' with HERVs; in that paper concluding that: "retroviruses and herpes viruses have complex interactions". That being said, not all results have arrived at the same conclusion.
I was also particularly interested to read the paper by Shuvarikov and colleagues  who seemed to suggest "HERV-H elements as a mechanism of deletion formation", as in genetic deletions. I might be making mountains out of molehills but their case report that HERV-H elements seemed to flank "recurrent, 3.4-Mb, de novo deletions of 3q13.2-q13.31" could potentially lead down some very important paths as intimated in a previous post. The fact that they mention autism as being part and parcel of some of their participant group description is likewise intriguing. As I've indicated in other posts on this blog where the term 'de novo' has been used, the uncertainty or chance finding of de novo now, might not be so in X numbers of years time. And the future may already be here  with schizophrenia in mind.
I'm going to stop there with this quite heavy going post and the chatter about HERVs and ADHD (or autism or anything else). Science is to quite a large extent still feeling it's way around the HERVs and their role - if any - in health and wellbeing. The reported link between the expression of HERVs and a condition as complicated as ADHD needs a lot more work on it before anyone can arrive at any firm conclusions. That being said, I find this to be a fascinating area of science which really does add a new layer of complexity to the whole genetics-environment relationship.
 Lavie L. et al. CpG Methylation Directly Regulates Transcriptional Activity of the Human Endogenous Retrovirus Family HERV-K(HML-2). J. Virol. 2005; 79: 876-883
 Perron H. et al. Human Endogenous Retrovirus Protein Activates Innate Immunity and Promotes Experimental Allergic Encephalomyelitis in Mice. PLoS ONE 8(12): e80128. doi:10.1371/journal.pone.0080128
 Wilson AS. et al. DNA hypomethylation and human diseases. Biochimica et Biophysica Acta. 2007; 1775: 138–162.
 Tugnet N. et al. Human Endogenous Retroviruses (HERVs) and Autoimmune Rheumatic Disease: Is There a Link? Open Rheumatol J. 2013; 7: 13–21.
 Balestrieri E. et al. Human endogenous retroviruses and ADHD. World J Biol Psychiatry. 2013 Nov 28. [Epub ahead of print]
 Santoni FA. et al. HERV-H RNA is abundant in human embryonic stem cells and a precise marker for pluripotency. Retrovirology. 2012; 9: 111.
 Mangeney M. et al. The full-length envelope of an HERV-H human endogenous retrovirus has immunosuppressive properties. J General Virology. 2001; 82: 2515-2518.
 Christensen T. Association of human endogenous retroviruses with multiple sclerosis and possible interactions with herpes viruses. Rev Med Virol. 2005 May-Jun;15(3):179-211.
 Shuvarikov A. et al. Recurrent HERV-H-Mediated 3q13.2-q13.31 Deletions Cause a Syndrome of Hypotonia and Motor, Language, and Cognitive Delays. Hum Mutat. 2013 Oct;34(10):1415-23.
 Bundo M. et al. Increased L1 Retrotransposition in the Neuronal Genome in Schizophrenia. Neuron. 2014. 2 Jan.
Balestrieri E, Pitzianti M, Matteucci C, D'Agati E, Sorrentino R, Baratta A, Caterina R, Zenobi R, Curatolo P, Garaci E, Sinibaldi-Vallebona P, & Pasini A (2013). Human endogenous retroviruses and ADHD. The world journal of biological psychiatry : the official journal of the World Federation of Societies of Biological Psychiatry PMID: 24286278